Neurogenic mechanisms in control of the rabbit iris sphincter muscle.

The iris sphincter muscle is supplied with cholinergic, adrenergic and substance P-containing nerve fibers, all with a possible role in the control of pupil size. The functional significance of the various nervous components in the rabbit iris sphincter muscle was examined in vitro and in vivo. The contractile response to electrical stimulation is composed of several contractions, occurring along different time scales. Single pulse stimulation produced an atropine-sensitive twitch. Pulse train stimulation revealed two successive atropine-sensitive twitches followed by a slow, long-lasting contraction, sensitive to Spantide, an antagonist of tachykinins such as substance P. A guanethidine- and phentolamine-sensitive contractile response to pulse train stimulation could be demonstrated in the presence of both atropine and Spantide. Only the Spantide-sensitive response could be completely exhausted by prolonged electrical stimulation. In vivo, neither the adrenergic nor the substance P-containing nerve fibres appeared to contribute to the miotic response to light since Spantide and guanethidine were without effect. This response was inhibited by atropine only.